题目

_________ has already been pointed out, grammar is not a set of dead rules. A. As                    B. It                C. That           D. What 答案：A将鸟卵的结构及其相应的作用用线连接起来．①卵壳                     A、保护作用②气室                     B、为胚胎发育提供养料③胚盘                     C、为胚胎发育提供氧气④卵黄                     D、起固定卵黄的作用⑤系带                     E、是进行胚胎发育的部位．